The Mannesmann-mandrel mill tube making process is widely adopted as a method for producing a seamless tube by hot working. In the mandrel mill rolling, the rolling is performed using multi-stand caliber rolls, which provide movements in an axial direction to a hollow shell while constraining/defining an outer surface of the hollow shell, and a mandrel bar, which constrains/defines an inner surface of the hollow shell. Therefore, the mandrel bar is an important tool which determines inner surface quality of the rolled hollow shell.
FIG. 1 is a view explaining a process of producing a mandrel bar used in mandrel mill rolling. Usually hot-work tool steels such as SKD6 and SKD61 (JIS standard) are used as materials for the mandrel bar. An ingot melted and prepared with a relevant chemical composition of a hot-work tool steel is bloomed and rolled, and a predetermined heat treatment is performed to obtain a bar material (a blank to be processed). Since the bar material is bent/crooked due to the heat treatment, the bends are corrected/straightened by a rotary straightener, and an outside machining device is used to machine the bar material into a predetermined outside diameter. And then, finish-polishing or surface treatment is performed to the surface of the workpiece, and plating is performed, whereby the process can repeatedly be applied to the mandrel mill rolling.
As described above, in the Mannesmann-mandrel mill tube making process, a thick hollow shell that is obtained by piercing through the heated round steel piece (billet) using a piercer is rolled into a thin hollow shell by plural roll-stands each comprising caliber rolls that are aligned as opposed to each other while the mandrel bar, which constrains the inner surface of the hollow shell, is inserted. The hollow shell to which the mandrel mill rolling is performed is re-heated if needed and rolled to a predetermined diameter to produce a final hot rolled product using a stretch reducer or a sizer.
Usually, in the mandrel bar employed in the mandrel mill rolling, a lubricating film comprising mainly solid-state lubricants is formed in advance on the surface of the mandrel bar to decrease frictional force incurred on the contact surface between the mandrel bar and the hollow shell, whereby generation of defects either on the tool surface or on the inner surface of the hollow shell is prevented.
However, the mandrel bar is repeatedly used, whereas the formed lubricating film disappears after one-time use of the mandrel bar in the mandrel mill rolling, so that the lubricating film needs to be formed again on the surface of the mandrel bar to use the mandrel bar in the next rolling. Accordingly, after the mandrel bar is once used in the rolling, the mandrel bar is cooled by water-cooling shower or the like, lubricants are coated over its surface, and the lubricants are completely dried to form the lubricating film.
As described above, the mandrel bar is made of a hot-work tool steel such as SKD6 or SKD61, and obtained by means of appropriate machining, quenching, and tempering. Since the surface of the mandrel bar bears a huge surface pressure and is exposed to a huge heat load during the rolling, the stable lubrication is hardly maintained. Therefore, the surface defects are likely to occur on the surface of the mandrel bar in association with repeated use of the mandrel bar in the mandrel mill rolling.
Conventionally, various countermeasures are studied against generation of surface defects on the surface of the mandrel bar. For example, Japanese Patent Application Publication No. 8-243610 discloses a life-extension method in which, after an outside surface of mandrel bar that is deteriorated in surface characteristics is polished by about 0.04 mm using a belter, rusting operation is applied on the surface of the mandrel bar at ambient temperature or by heating the mandrel bar to 100° C., and the mandrel bar is reused as the mandrel bar of like size.
However, the intended mandrel bar of Japanese Patent Application Publication No. 8-243610 is the one which premises a scaling treatment. Since recently the mandrel bar to which a hard Cr plating treatment is performed is mainly used to improve a wear-resistant property, the life-extension method disclosed in Japanese Patent Application Publication No. 8-243610 cannot be applied to such a case. Surface roughening is concerned in the mandrel bar to which a scaling treatment is performed, while the generation of surface defects is concerned in the mandrel bar to which the Cr plating treatment is performed.
Japanese Patent Application Publication No. 07-214116 proposes a mandrel bar for seamless tube rolling in which, even if wear or surface defects are generated on the surface of mandrel bar, wherein the mandrel bar is not partially disposed, and is configured such that a body portion of the mandrel bar is covered with plural sleeves to thereby allow the mandrel bar to be reused by making it possible to exchange this sleeve(s) when needed upon generation of damages thereon, thus enabling to improve cost performance of the tool, or tool costs per production unit.
However, in the mandrel bar proposed in Japanese Patent Application Publication No. 07-214116, since the body portion of the mandrel bar is covered with the plural sleeves, the production cost of the mandrel bar is largely increased, and a serious accident such as deformation/distortion and/or coming-off of this sleeve(s) is possibly induced during the rolling.